The use of solid state lasers (laser diodes) in digital printers or scanners has many advantages including compact size, high power, and low cost. These light sources have a disadvantage because they require a relatively high magnification optical system making such systems sensitive to changes in the axial position of the light source. The reason for the high magnification is explained as follows.
Typically, the light emitting region of the laser diode is focused onto a light sensitive medium by an optical system. Through a variety of means, the focused spot can be scanned across the light receiving medium and turned off and on at a high rate. The light sensitive medium is exposed and the image is built up of many small spots or pixels. In a digital printer it is important to maintain a constant writing spot size, because image artifacts can appear if it is digitized with a varying size pixel. In this and the remaining description, we will define the writing spot or pixel diameter to be that diameter at which the intensity of the light is e.sup.-2, approximately 0.1353, of the peak intensity.
The high magnification referred to above results from the fact that the light emitting region of a laser diode is very small compared to the writing spots commonly used. A typical pixel size used is that for a 300 spot per inch printer which has an e.sup.-2 diameter of approximately 0.08 mm. The light emitting area of laser diodes ranges approximately from 0.5 to 2 microns e.sup.-2 diameter in its smallest dimension and 2 to 6 microns in its largest dimension. The light emitting area is the contour of the light intensity at which the intensity is e.sup.-2 of the peak intensity. Required is an optical system between the laser diode and image receiving medium having a magnification from 10 to 160 times. For example, a 1 micron laser diameter emitting area would need a magnification of 80 to make an 80 micron e.sup.-2 diameter pixel.
A high magnification optical system has the characteristic that its image location along the optical axis is sensitive to the object position change. The longitudinal magnification which is the magnification of an infinitesimally short line along the optical axis is the square of the transversal magnification. For a system which has 80 times magnification as above, the longitudinal magnification would be 6400. This means that if the object shifts along the optical axis by 0.001 mm or 1 micron, the image will shift by 0.001 times 6400 which is 6.4 mm. A laser shift of this amount would likely cause the writing spot of a laser printer to be out of focus and increase in diameter by an unacceptably large amount. Therefore the space between the laser and the optical system must be tightly controlled. On the other hand, laser printers often need to operate in an environment whose temperature changes over a wide range. These uncontrolled temperature changes will cause the material holding the laser with respect to the optical system to expand and contract. Thermal expansion changes leading to 0.001 mm changes can very easily occur.
Normally laser printers are designed so that the Gaussian laser spot or waist is located on the image receiving medium. A "waist" is defined as the position where the minimum focused spot occurs. In general this waist position is different than the location of the geometrical optics determined conjugate point, but it can be calculated by well known procedures. The optical system of a laser printer takes the input laser waist on the object side of the optical system and creates the waist of the desired size on the output or image side of the optical system. The ratio of the image side waist to the object side waist is the transverse magnification.
One approach to stabilizing the laser diode axial position with respect to the optical system is to mount the laser and first lens of the optical system, usually a "collimator", in a thermally stable mount or "athermal head". The athermal head is carefully designed with different materials to achieve compensation so that the axial position of the lens with respect to the collimator is very stable. This approach becomes very difficult and expensive if the desired axial space control is in the range of a few tenths of microns. Also, it may not be desired to actually control the physical space between the laser and collimator lens itself because if the lens changes it paraxial characteristics with temperature, its first principal plane may shift with respect to its actual external lens surfaces. The requirement then is to stabilize the distance from the laser input waist to the first principal plane of the collimator. This means that the lens paraxial properties would have to be completely characterized thermally in order to predict the motion of the first principal plane when the temperature changes. An example of this athermal head approach is disclosed in U.S. Pat. No. 4,948,221 issued Aug. 14, 1990 to inventor Yates. This invention, however, does not take into account the optical changes of the lens with temperature.
Another approach is to actively compensate for thermal changes by sensing the image waist motion and adjusting a lens position to place the waist back to the correct position. This requires a method to detect the waist position at the image, a way to relay this information, a method to move a lens and accept the waist error signal, and a moveable lens. This results in added complexity and cost, but it has the advantage of compensating for athermal expansion and contraction throughout the optical system and not just those parts near the laser. An example of the technique is published in European Patent Application 0323850.